A conventional fluid coupling includes first and second coupling members having respective fluid paths communicating with each other, an annular gasket interposed between butted end surfaces of both the coupling members, and a screw means that couples the coupling members to each other, and the butted end surface of each coupling member has an annular sealing protrusion and an anti-overfastening annular protrusion protruded further than the sealing protrusion (for example, Patent Document 1).
In the fluid coupling, the annular sealing protrusions come into contact with the gasket to deform the gasket, thereby obtaining required sealing property, and the anti-overfastening annular protrusions prevent fastening exceeding proper fastening. At proper fastening, the anti-overfastening annular protrusion of one coupling member is opposed to the anti-overfastening annular protrusion of the other coupling member via a retainer or not via the retainer, and the anti-overfastening annular protrusions do not exert any force to each other or do not exert a substantial force to each other. To prevent overfastening, at further fastening, the anti-overfastening annular protrusion of one coupling member and the anti-overfastening annular protrusion of the other coupling member butt against each other via the retainer or directly, increasing a resistance to fastening, thereby preventing excessive fastening.